when I run my custom map, I open console and see some red color warnings:
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"Map dm_backyard missing stringtable dictionary, don't ship this way!!!
Run with -stringtables on the command line or convar
stringtable_alwaysrebuilddictionaries enabled to build the string table"
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I'm not 100% sure, but I believe it has something to do with the navigation mesh. I seem to recall seeing this message on my map before it had any mesh, and I don't think I've seen it since generating one.

Appears to be a txt file with a list of some models, sounds and things in it. No idea what for. I think its built automatically and put into the .bsp when you run the map for the first time. So your supposed to run the map least one time for giving it out.

There are numerous times when the engine needs to transmit a string to clients (e.g. the name of a sound to play). Instead of literally transmitting the potentially quite long string, it transmits an numeric index in the map's embedded string table.

Since the client and server map files have to match it's guaranteed that the right string will be found by the client.